Supporters say a financial transaction tax (FTT) would deter market speculation. But the industry and other critics are expanding efforts to stop the proposal, saying it would weaken markets and hurt small investors, especially Americans trying to save for retirement. Retirement money is often invested in mutual funds that trade frequently to maximize returns.

The Investment Company Institute (ICI), which represents mutual funds, wrote to House Budget Committee ranking member Chris Van Hollen (D-Md.) last week, urging him not to include a transaction tax in any Democratic budget plan. Van Hollen floated the idea last year to help pay for middle-class tax relief.

“Quite frankly, an FTT is a terrible idea,” the group said. “It would harm all investors, especially middle-income American workers saving for retirement.”

The 2016 election also makes it an important moment for the financial industry to speak out against the proposals, said Micah Green, who leads Steptoe & Johnson’s financial services practice.

“Because of the increased rhetoric on the campaign trail against Wall Street and the financial markets, this is an issue that shouldn’t be ignored,” he said.

This is the first presidential election in which a financial transaction tax is being discussed, said Steven Rosenthal, a senior fellow at the Urban-Brookings Tax Policy Center. It’s gaining attention because the 2008 financial crisis “still echoes” with voters, he said, and because candidates are looking for other sources to fund programs.

Sanders would use the tax on trading to make public colleges tuition-free. Legislation the Vermont senator proposed in the Senate would tax stock transactions at 0.5 percent, bond transactions at 0.1 percent and derivative contracts at 0.005 percent. Individuals with incomes of $50,000 or less and married couples with incomes of $75,000 or less could receive a credit against the tax.

“During the financial crisis, when Wall Street’s greed and illegal behavior nearly destroyed our economy, the middle class of this country bailed out Wall Street. Now, it is Wall Street’s turn to help the middle class of this country,” Sanders said in a January speech.

Taxing high-frequency trades would be a more narrow approach than Sanders’s proposal. Clinton’s tax would focus on traders who place orders and then cancel trades — behavior critics say can distort markets — but she has not provided details, Rosenthal said.

The Congressional Progressive Caucus’s fiscal 2017 budget proposal, released earlier this month, also includes a tax on securities trades.

The group would tax stock transactions at 0.25 percent, bond transactions and foreign-exchange transactions at 0.004 percent, futures and swaps at 0.01 percent, and option premiums at 0.25 percent per year to maturity.

“The tax would reduce reckless speculation that adds uncertainty while driving up prices of key commodities,” the caucus said.

The idea of a financial transaction tax isn’t new, and the financial industry has fought against the idea for years. The tax isn’t likely to be enacted this year, but critics say the visibility of the issue makes it important for them to get their message across.

“Any time an FTT is raised as a policy option, it is important to remind policymakers and the public of the financial harm this new tax would cause for millions of mutual fund shareholders saving for retirement, education and other financial goals,” ICI spokesman Matthew Beck said.

The Modern Markets Initiative, an advocacy group for high-frequency trading firms, has been writing op-eds to speak out against the tax proposals and is talking to other financial groups and policymakers about the issue, the group’s chief executive officer, Bill Harts, said.

“It’s important enough, and enough people are talking about it, that it deserves scrutiny,” he added.

“Since [high-frequency trading] has gained prominence over the past 20 years, our industry has focused on lowering investor costs. We measure costs like spreads in fractions of a cent, and that focus has led to the best markets ever from the investor’s standpoint, and a competitive advantage for our country,” he said.

“We are mystified as to why any politician would want to give away that advantage.”

Several countries in Europe also have financial transaction taxes, and a group of European Union members is taking steps toward coordinating on the tax, but it’s unclear if they will succeed.

The Financial Services Roundtable hasn’t done any specific advocacy on transaction taxes lately. But Francis Creighton, the Roundtable’s executive vice president of government affairs, said he is concerned the proposals could find their way into future legislation.